The present invention relates to an inexpensive method for impregnating wood with metal salts, and more particularly, to a method for impregnating wood with preservative and fungicidal treatments.
In order to prevent decay of wood and timbers, and thereby increase their life, it is common practice to impregnate the wood or timbers with a preservative such as creosote dissolved in liquid aromatic hydrocarbons, mixtures of organic compounds which are dissolved or dispersed in water, or certain compounds which are dissolved in petroleum distillates. The protection afforded by the application of these materials is dependent upon deep and reasonably uniform penetration into the wood or timber by the preservative material. It also is desirable that the treatment be effective without a significant change in the original dimensions and surface texture of the wood or timbers.
The subject of wood treatment and wood preservation is discussed in some detail in the two volume treatise entitled "Wood Deterioration and its Prevention by Preservative Treatments", Darrel D. Nicholas, Editor, Syracuse Wood Science Series 5, Syracuse University Press, Syracuse, N.Y., 1973. Among the examples of wood preservatives described therein are various creosote compositions, pentachloro-phenol, copper naphthenate, copper-8-quinolinolate, organotin compounds, organomercury compounds, zinc naphthenate, chlorinated hydrocarbons, ammoniacal copper arsenite (ACA), acid copper chromate (ACC), zinc salts such as zinc chloride, zinc oxide and zinc sulfate, chromated copper arsenate (CCA), etc. In Volume II, Chapter 3, pages 279-298, processes and equipment for treating wood are discussed. The pressure treatment is described as the most effective method of protecting wood against attack of decay, insects, fire, etc. Non-pressure treatments also are discussed in this chapter. Dipping is suggested primarily as a satisfactory surface treatment although some penetration is observed. Another non-pressure technique is the diffusion process with unseasoned wood. The author indicates the process requires long treating periods because of slow diffusion rates.
While the literature on the subject of such wood treatments is extensive and covers a period of at least 100 years, most of the procedures which have been described for treating wood with preservatives and resin components, including pressure treatments, do not result in extensive uniform impregnation of the material into the heart of the wood and/or the procedures require a long period to effect the penetration. The problem is particularly acute when treating some types of green or incompletely seasoned wood such as soft pine.
The use of liquid aromatic hydrocarbons for preparing impregnating solvents imparts to the wood strong odors and leaves the wood with a surface which is oily and difficult to paint. Moreover, liquid aromatic hydrocarbons are flammable materials requiring special handling and safety precautions which add to the cost of the wood treatment.
Wood treated with organic preservatives dissolved in petroleum distillates have the same disadvantages as wood treated with the aromatic hydrocarbons. Using lower boiling petroleum distillates, such as mineral spirits, as the solvent, fails to eliminate the disadvantages completely. Prolonged air seasoning after treatment is frequently required to permit sufficient evaporation of the solvent if the wood is to be painted. During this period of air seasoning, a portion of the preservative can migrate to the surface of the wood with the solvent, and thus, the retention of the preservative into wood is reduced below that contemplated by the treatment.
One technique for utilizing aqueous systems of polyhalophenols is described in U.S. Pat. No. 4,090,000. Briefly, the method involves the use of an aqueous solution containing a water-soluble salt of the polyhalophenol and an acid forming material which can undergo a reaction in the solution to liberate an acid which displaces the polyhalophenol from said salt after the solution is impregnated into the wood.
Regardless of which impregnating solution is employed, the most common commercial procedure for impregnating wood involves subjecting wood to the preservative under relatively high pressures such as 150 to 200 pounds to the square inch for a substantial period of time such as from one hour to 24 hours. The process also may require relatively high temperatures such as from about 75.degree. C. to about 90.degree. to 95.degree. C. Although increases in pressure tend to increase the amount of preservative absorbed by the wood, it may cause the penetration to be erratic or uneven. Moreover, the application of pressure can cause compression of the outer layers of the wood, particularly after wood is weakened and softened by steaming. The collapse of the wood cells is likely to occur especially when relatively soft, unseasoned wood of low specific gravity is being treated. On collapse of the wood cells in an area, there is formed a relatively impenetrable layer which restricts or even completely blocks the flow of preservatives into the interior of the wood.
It also has been suggested to improve the method of pressure treatment by first subjecting the wood to a vacuum treatment. Examples of prior art patents describing methods of impregnating wood utilizing a vacuum followed by pressure include U.S. Pat. Nos. 2,668,779; 3,200,003 and 3,968,276.
U.S. Pat. No. 3,677,805 describes a modification of the pressure treatment. In this procedure, the wood is immersed in a treatment liquid inside a pressure vessel, and the pressure is increased to operating pressure whereupon the contents of the vessel then are subjected to the action of a pulsating pump which provides sinusoidal pressure pulses within the vessel. In other words, pressure pulses are applied repetitively in modulated amplitude to provide variable pressure peaks above and below the ambient pressure maintained in the pressure vessel. This procedure requires equipment which includes a pulsating pump operating into a pressure vehicle equipped with a pressure release means.
The above-described prior art represents a small sampling of the suggestions which have been made for treating wood with preservative materials to prevent decay. In spite of these many suggestions made in the prior art, there continues to be a need for an inexpensive, safe, non-toxic treatment which is effective and which results in the uniform penetration of the preservatives and other chemicals to the core of the wood.